MKIII conversion on a 70 750

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Aug 11, 2008
Hi all. I was referred here by one of your members to ask for your assistance with my dillema. Though it probably wont be most of you guys' cup of tea, I am building a chopper using what is titled as a 70 Norton 750. I purchased the frame and engine last winter and have been working on it ever since. What I have run into is a problem converting to the MKIII isolastics. I attempted to do a search on this site and others but haven't seen the particular problem I have. I should mention that I have no experience with Norton Motorcycles. I'll give you a rundown of what I'm experiencing with pictures:

This is the bike in a semi-mock up stage:

MKIII conversion on a 70 750

Here are the frame tabs that the bolt runs through for the rear mount of the transmission cradle:

MKIII conversion on a 70 750

Here are those same tabs in relation to the MKIII isolastic I purchased and this is where my problem is...It would appear to me that the conversion is not possible due to the fact that the tabs are too narrow to accomodate the isolastic mount. When I dissassembled the rear mount, the only parts in the tube were the outer metal bushings with the flanges on them and inside there was only one rubber. No where was there any shims or the like and the rear mount fits perfectly between those two frame tabs:

MKIII conversion on a 70 750

Here's the new isolastic in relation to the motor itself:

MKIII conversion on a 70 750

And, lastly, the front mount appears as though it will work flawlessly as it fits perfectly between the front tabs:

MKIII conversion on a 70 750

SO...With all of that, can anyone tell me how this is possible? Is the rear mount that came with the bike missing parts and are the frame tabs welded too narrowly? What is the inside distance of a factory 70 Norton? Any assistance is greatly appreciated.
Also for those interested here's a then and now picture and some more details on the bike. The bike originated on the east coast and was purchased in 1986 by the fella that I bought it from. In 86, he had the engine overhauled and drove the bike about 100 miles prior to tearing it down again to install footpegs as opposed to just using highway bars. The bike lay in pieces and the military moved him so the bike was in boxes for the next 20 years. He moved it with him to Wyoming where I found it and I paid him today what he had in just the engine overhaul 20 years ago. So...with any luck the motor will be in good shape. It has high compression pistons and a Joe Hunt Mag. Hopefully I can pull a wheel stand. Haha! The biggest changes I've made aside from the obvious cosmetic changes, ie: seat, handlebars etc... Are the fact that it now has disc brakes front and rear. I traded the rear Morris mag wheel for some other parts and replaced it with an MKIII hub laced to a 16" harley wheel and stainless spokes. The front master cylinder is a honda and is a smaller diameter piston bore than the factory Norton so hopefully that helps with the "wooden" feel of the factory setup that I hear about. The rear master cylinder will be a Harley unit but I've yet to fabricate that setup. I am also running a set of British Cycle Supply forward controls that I fabricated to bolt to the Norton engine and front motor mount. If anyone is interested in this particular project let me know, I'll send you some pics and details. The rear fender is a '36 Ford spare tire cover and the taillight is a '39 Chevy. The tank is a Wassell banana. In order to finish the bike, I need to figure out my rear master cylinder setup, do some fab work on my oil tank, plumb the brake lines, install an oil filter setup, wire it, and figure out this isolastic deal. Other than that, it has come a long ways...haha! Let me know what you think of the bike and what I can do with this isolastic disaster.



MKIII conversion on a 70 750

Now, minus forward controls installed:

MKIII conversion on a 70 750
The engine cradle looks unmodified. A rigid rear end on a Commando presents problems because normally the swing arm and rear wheel assembly is mounted to the power unit. By mounting the rear wheel to the frame and leaving the engine suspended on rubbers, you have the risk of chain alignment problems which are what led Norton to create their strange design in the first place.

Aren't most Commando chops mounted without isolastics ?
That's a totally modified custom frame, it obviously was desined with the older style shim-type rear mount setup.

I have a Norton chopper frame that used a real simple saddle setup to bolt the tranny cradle in, retaining the isolastics and the chain alignement. As it turns out, I just might build it back up as the chopper it was, they seem to fetch decent money.
If I remember correctly Uncle Bunt in the UK built a hard tail Commando Chopper with the ISOs fitted and it shed it's chain.

Thanks for the responses. I stated before, I know nothing of Norton project to start eh? I'm a slow learner...anyway, what I gather is that the rear isolastics rely on the swingarm for support? That really doesnt make any sense to me at all, even when looking at the maintenance and repair manuals I have, I just dont see why or how that works...anyone have a link to an article I can read on the matter? On my engine it appears as though they retained the front isolastics and solid mounted the rear. I suppose I could set it up like this again as that is how it was designed to fit into the custom frame.

Thanks again
Actually, the transmission cradle is the "heart" of the isolastic chassis.

The tranny cradle houses the swingarm pivot, rear isolastic pivot, and rear engine mounting points, the front engine mount houses the front iso picot and front engine mounting points.

When you mount the engine and tranny cradle in the frame, along with the upper head steady (whichever one), you (ideally) get a stable platform for the engine to vibrate away with fore/aft allowance in an arc from the rear pivot. Again, ideally, with good parts, there is very little side to side and torsional movement.

To this assemblage, you mount the swingarm. If I read it all correctly, you end up with slightly variable rear suspension geometry based on the fact that the swingarm pivot point actually has a slight range of motion in some form of near-random elliptical pattern.
Tango, If you read the inputs from Panic in that thread you will see that he points out that it isn't a problem to figure how to ISO mount it it is a problem to make the rear sprocket to move in unison with the motor. The turnbuckle bearings can do this allowing motion one way and not the other. Wendy can make you some ISO's that can do this and he has built a chopper from a Norton. He goes by Bill on this forum. Basically you need a turnbuckle to keep the motor study against the the pull of the chain that isn't that hard. All the know how has been out there for some time.
Actually, on a hardtail, it's not nearly as important becasue the relationship only varies by the amount of the engine's vibration. The relationship between the swingarm's center of rotation and the engine sprocket's center is no longer an issue.
Paul, think about the engine pulling back out of line as it feels the torque of trying to turn the rear wheel. The sprockets won't stay in line. The chain will fly off the sprockets. One threaded link in line with the chain could prevent it from happening though.
So...with all of that said, and given panic's input on the BB forum, I understand that it is necessary to install some sort of turnbuckle system that is parrallel with the chain, and that prevents that back-left torsion of the engine that will dissalign the that correct?
Yes, you're on your own engineering it. But that's the basics. There are people who can help you think about it though. Buell's mount up using this idea you might take a long look there. Windy (bill) has some pictures of his system on a bike he might send you.
So even with a new set of mounting rubbers in a shim type system, you get enough torque misalignment to throw chains? Yikes.
wow, you guys have been great. So I fully understand what my setup is in comparison to a factory swingarm Norton, with a swingarm Norton, there is enough give in the swingarm to allow "wiggle room" so the sprockets essentially stay true to each other. Looking at my setup, I really see no reason why a panhard bar/turnbuckle setup mounted to the rear left tranny cradle plate and tied into my frame crossmember that the fender mounts to would be all that difficult....Should I do one on each side of the tranny cradle? I'll shoot bill a PM and see if he can chime in....

Thanks again for all of the input!
The chopper that I built kept the swingarm so I did not run into the problems that you are facing. you might look at AMR with how they did there featherlastic.
I think you are on the right track by having to limit the rearward pull that the chain will put on the mounts.

Thanks again for all of the input folks. I tracked down a local fellow who built a Norton chopper that is a hardtailed original frame. His claim is that he has never had any problems with the chain on his bike. He runs an O-ring chain and keeps it very tight. He's running a sportster rear on it. He built it 10 years ago and states he's put quite a few miles on it, drag raced it, and hasn't yet had a single problem with regards to the chain. I might just throw this baby together and see what happens haha! I'll keep ya'll posted.
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